The python community is sunsetting the support of python2 (https://python3statement.org/)

and we should follow these activities to drop python2 in 2020.

This patch removes python2 from Cantor. Old worksheets created with python2 will be moved
to using python3 on load - there is a info message box to the user to notify him/her about this.

Differential Revision: https://phabricator.kde.org/D27331